Polymeric Sol Gel Research Articles

Polymeric Sol-Gel Transition with the Diverging Correlation Length Verified by Small-Angle X-ray Scattering.

Sol-gel transitions of polymers are pivotal phenomena in material science, yet the critical phenomenon of structure during gelation has remained unclear. Here, we investigated the sol-gel transition of a fluorous polymer, poly(vinylidene fluoride-co-hexafluoropropylene), in a blend of two ionic liquids. This system features a quite high amount of cross-linker and binding sites with ion-dipole interactions between the cation and C-F dipoles, thereby facilitating easy exchange of the cross-links. Changing the mixing ratio of the two ionic liquids enabled tuning the ion-dipole interactions and inducing sol-gel transition. Notably, the correlation length and molar mass, obtained by small-angle X-ray scattering, diverged at the gelation point. Moreover, the derived critical exponents (ν = 0.85 ± 0.05) aligns remarkably well with the prediction from percolation theory (ν = 0.88). To our knowledge, this is the first report on the evident divergence during polymeric gelation by small-angle scattering and the verification of the critical exponents of the percolation theory.

A Pt-free pristine monolithic carbon aerogel counter electrode for dye-sensitized solar cells: up to 20% under dim light illumination.

In this work, pristine carbon aerogels (CAs) were used as Pt-free counter electrodes (CEs) in dye-sensitized solar cells (DSSCs) by varying the molar ratio of their precursors. Pristine mesoporous CAs with controlled resorcinol (R)/formaldehyde (F) and resorcinol (R)/sodium carbonate (C) molar ratios were successfully prepared. The as-prepared CAs were synthesized via a polymeric sol-gel reaction and were labeled as CA-O, CA-Q, CA-F, CA-C, and CA-G. The DSSCs using the as-prepared CA-C CE gave the best power conversion efficiency (PCE, η), 9.08 ± 0.01%, among all the CA CEs. The CA-C CE is further applied to an indoor T5 light source system with an impressive η value of 20.1 ± 0.60% at 2.18 mW cm-2 (T5 lamp with 7000 lux). Moreover, the hardness of CA-C CE is 3.01 GPa (Brinell hardness test), which is comparable to that of the FTO/glass substrate. As a result, the CA-C CE shows great potential to replace traditional CEs based on the Pt/FTO/glass in DSSCs.

Fabrication of durable superhydrophobic and superoleophilic cotton fabric with fluorinated silica sol via sol–gel process

ABSTRACTSuperhydrophobic and superoleophilic cotton fabric was successfully prepared with fluorinated silica sol via a facile sol–gel method. A fluorinated polymeric sol–gel precursor (PHFBMA‐MTS) was synthesized via free‐radical polymerization by using hexafluorobutyl methacrylate (HFBMA) in the presence of (3‐mercaptopropyl)trimethoxysilane (MTS) as the chain transfer agent, which led to the formation of fluoropolymer with alkoxysilane end groups. Then the fluorinated silica sol was prepared by introducing PHFBMA‐MTS as the co‐precursor of tetraethylorthosilicate (TEOS) in the sol–gel process with ammonium hydroxide as the catalyst, which was then used to fabricate superhydrophobic and superoleophilic fabric coatings through a simple dip‐coating method. The coated fabrics showed superhydrophobic property with a high water contact angle of 154.1° and superoleophilic property with an oil contact angle of 0°. Moreover, the coated fabrics still kept superhydrophobicity even after ultrasonic treatment, as well as for organic solutions, acidic solutions. Thus, the coated fabrics were successfully applied to separate oil–water mixture with separation efficiency up to 99.8%. More importantly, the separation efficiency had no significant change after 20 cycles of oil–water separation. These present a simple, low‐cost, and durable approach to achieve industrialized application of coated fabrics in oil–water separation. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47005.

Synthesis of nanostructured titania/zirconia membrane and investigation of its physical separation and photocatalytic properties in treatment of textile industries wastewater

Nanostructured TiO2/ZrO2 composite membranes with varying compositions were obtained by sol–gel technique. The influence of 0–30 mol% zirconia doping on microstructure, water permeability, photocatalytic and physical separation properties, removal of methyl violet of textile industries wastewater and thermal and mechanical stability of titania/zirconia composite membranes was described. Firstly, alumina supports were coated with TiO2 intermediate layers using the colloidal sol–gel route. The TiO2/ZrO2 composite sols were prepared via a polymeric sol–gel method and dip-coated on TiO2 intermediate layer. The samples were characterized by DLS, TG-DTA, XRD, FTIR, BET-BJH, UV–visible, SEM, TEM and AFM. It was shown that zirconia retards the phase transformation of anatase to rutile until at least 700 °C. The minimum pore size and maximum surface area obtained were 1.2 nm and 153 m2/g, respectively, attributed to the sample with 20 mol% zirconia. The mechanical strength of titania membranes was significantly improved by addition of zirconia. The most methyl violet removal efficiency obtained, with and without UV-irradiation, is 80.8 and 72.6%, respectively, attributed to the sample with 20 mol% zirconia.

Ce3+ and Eu3+ emissions in YAG via a core-shell strategy for warm white LED lighting

YAG:Ce and YAG:Eu sols were synthesized by a polymeric sol-gel route. The crystallization temperature of YAG was determined by X-ray diffraction as a function of the calcination temperature, revealing that YAG starts to crystallize directly from the amorphous phase at 800 °C. The effects of the thermal treatment and the dopant amount on the photoluminescent properties were studied, observing the highest emission after calcination at 1000 °C for 1 h in both cases and with a concentration of 1 and 3 mol% of Ce and Eu, respectively. Core-shell materials were prepared by dipping YAG:Ce or YAG:Eu sintered pellets into the synthesized sols and then, these materials were calcined at 1000 °C for 1 h. An effective energy transfer from Ce to Eu was observed in the sample YAG:Eu (core)–YAG:Ce (shell) when blue light (λ = 465 nm) is used as excitation source. This wavelength excites the Ce but not the Eu; however, in the photoluminescence emission spectrum, the bands associated to both ions can be clearly detected, confirming that the core-shell strategy is a good method for the preparation of warmer white LEDs.

Tuning sol size to optimize organosilica membranes for gas separation

Abstract A series of organosilica sols are prepared by the polymeric sol–gel method using 1, 2-bis(triethoxysilyl)ethane (BTESE) as the precursor. Particle size distributions of the BTESE-derived sols are systematically investigated by carefully adjusting the synthesis parameters (i.e., water ratios, acid ratios and solvent ratios) in the sol process. In certain conditions, increasing the water ratio or the acid ratio tends to cause larger sol sizes and bimodal particle size distributions. However, higher solvent ratios lead to smaller sol sizes and unimodal particle size distributions. The organosilica membranes prepared from the optimized sols show excellent H2 permeances (up to 4.2 × 10− 7 mol·m− 2·s− 1·Pa− 1) and gas permselectitivies (H2/CO2 is 9.5, H2/N2 is 50 and H2/CH4 is 68). This study offers significant insights into the relationship between the sol synthesis parameters, sol sizes and membrane performance.

New red-shade environmental-friendly multifunctional pigment based on Tb and Fe doped Y2Zr2O7 for ceramic applications and cool roof coatings

A new red-shade environmental friendly ceramic pigment with the general formula Y2−xTbxZr2−yFeyO7−δ [x = 0, 0.25 and y = 0.15, 0.25, 0.35] was prepared by a polymeric sol-gel process and after calcination at 1300 °C for 12 h. The solid solution range with defect fluorite structure was determined by XRD. The presence of terbium ions promoted the incorporation of iron ions in the structure. The evolution of the colour was studied by UV/Vis absorption measurements and CIELab chromatic coordinates were determined. The colour was related to the oxidation states of the terbium and iron ions studied by XPS and photoluminescence. Pigments presented the best chromatic coordinates when the amount of Fe(III) and Tb(IV) ions were higher. The sample with good red colouration (x = 0.25, y = 0.35) was tested with an industrial frit, presenting slightly better chromatic coordinates to those of the commercial ceramic pigment currently used. The pigment also showed an optimal NIR reflectance to be applied as “cool pigment” for roof isolation.

Pd-doped organosilica membrane with enhanced gas permeability and hydrothermal stability for gas separation

A Pd-doped organosilica membrane based on bis(triethoxysilyl)ethane is successfully developed by the polymeric sol–gel method. Its microstructure, chemical composition, and separation performance are compared with those of the undoped organosilica membrane. Gas adsorption analysis indicates that the Pd-doped organosilica membrane has larger micropores compared with the undoped organosilica membrane. The gas permeation results show that the Pd-doped organosilica membrane has much higher gas permeances than the undoped organosilica membrane due to the enlarged micropores after Pd-doping. The Pd-doped organosilica membrane also exhibits a significantly improved hydrothermal stability. The enhanced hydrothermal stability can be explained by the mechanism that Pd particles act as inhibitors and prevent the formation of mobile silica groups (e.g., Si–OH) under steam condition. Metal-doping (e.g., Pd-doping in this work) may offer a new approach to develop high performance membranes with enhanced gas permeances and hydrothermal stabilities in gas separation applications.

Fabrication and characterization of TiO2/ZrO2 ceramic membranes for nanofiltration

TiO2/ZrO2 ceramic nanofiltration membranes are successfully fabricated through the polymeric sol–gel route followed by the dip-coating technique. Disk type α-alumina supported mesoporous γ-alumina (pore size: 5–6 nm) is employed as the support in dip-coating. The unsupported and supported composite ceramic membranes are systematically characterized and evaluated in terms of phase composition, chemical stability, gas adsorption, molecular weight cut-off (MWCO), membrane pore size, water flux and salt rejection. It is found that the TiO2/ZrO2 ceramic membranes have amorphous phase at 400 and 500 °C, suggesting the high thermal stability. The fabricated membranes have the MWCO of 620–860 Da, corresponding to the membrane pore size of 1.2–1.5 nm. Relatively low water permeability can be attributed to the low microporosity of the membrane. Donnan exclusion is the dominant transport mechanism of the NF membrane in the single-component system, and salt rejection is closely related to the hydration properties of the ions (e.g., the hydration radius).

Synthesis of polymeric fluorinated sol–gel precursor for fabrication of superhydrophobic coating

A fluorinated polymeric sol–gel precursor (PFT) is synthesized by copolymerization of 2,3,4,5,5,5-hexafluoro-2,4-bis(trifluorinated methyl)pentyl methacrylate (FMA) and 3-methacryloxypropyltrimethoxysilane (TSMA) to replace the expensive long chain fluorinated alkylsilanes. The fluorinated silica sol is prepared by introducing PFT as co-precursor of tetraethyl orthosilicate (TEOS) in the sol–gel process with ammonium hydroxide as catalyst, which is then used to fabricate superhydrophobic coating on glass substrate through a simple dip-coating method. The effects of PFT concentrations on the chemical structure of the formed fluorinated silica, the surface chemical composition, surface morphology, wetting and self-cleaning properties of the resultant fluorinated silica coatings were studied by using X-ray powder diffraction (XRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrophotometer (XPS), scanning electron microscopy (SEM) and water contact angle measurements (WCA). The results show that the fluorinated silica sols are successfully obtained. The size and size distribution of the fluorinated silica particles are found greatly dependent on the concentration of PFT, which play a crucial role in the surface morphology of the corresponding fluorinated silica coatings. The suitable PFT concentration added in the sol–gel stage, i.e. for F-sol-1 and F-sol-2, is helpful to achieve both the low surface energy and multi-scaled microstructures, leading to the formation of the superhydrophobic coatings with bio-mimicking self-cleaning property similar to lotus leaves.

Synthesis and Characterization of LaCoxFe1-xO3 (0≤x≤1) Nano-Crystal Powders by Pechini Type Sol–Gel Method

In the present work, the authors report the results of the influence of the cobalt ions which partly substitute the Fe ions in the LaCoxFe1-xO3, where x = 0; 0.2; 0.4; 0.6; 0.8; 1 on the structural properties. A polymeric sol-gel (Pechini) technique has been used to prepare nano sized powders of LaCoxFe1-xO3 semiconducting oxide. Thermal decomposition of polymeric intermediate was studied via DTA-TG analysis. The phase composition, crystal structure and morphology of the samples were studied by XRD, SEM, and TEM. Considering the DTA-TG results, the addition of cobalt to the system led to a shift toward lower temperatures. In comparison with LaFeO3, the intensity of LaCoxFe1-xO3 (0<x≤1) peaks declined markedly, although the full widths at half maximum were far broader. The average crystallite sizes of the final powders that were almost spherical varied from 28 to 11 nm, resulting from an increase in the cobalt amount.

Raman investigation of germanium- and phosphorus-doping effects on the structure of sol–gel silica-based optical fiber preforms

This works reports the structural investigation of homogeneously doped sol–gel-derived silica optical fiber preforms. Crack-free germanosilicate (2.7 wt% Ge) and phosphosilicate (2.5 wt% P) glasses, suitable for optical fiber fabrication, have been prepared using the polymeric sol–gel route combined with the sol-doping technique. Space homogeneity of the doping oxide (GeO2 or P2O5) in the cylindrical preforms has been checked by chemical analysis. The structure of these glasses have been studied using Raman spectroscopy and compared to pure silica glasses. It is shown that phosphorus has a much more sensitive effect on the glassy structure than germanium, both at small and medium scales. Indeed, in the low-wavenumber region (7−100 cm−1), the phosphorus-doping was found to affect the intensity of the boson peak, suggesting an enhancement of heterogeneity at the nanometer scale. At a shorter scale, both germanium- and phosphorus-doping were found to reduce the number of three- and four-membered rings in the silica glass network.

Interlayer-free microporous cobalt oxide silica membranes via silica seeding sol–gel technique

This work shows for the first time that interlayer-free cobalt oxide silica membranes can be produced via a novel silica sol–gel seeding process. The silica seed, synthesized from the Stӧber process, had an average particle size of 70nm, which is incorporated into the polymeric cobalt silica sol with a particle size of less than 1nm. Calcined cobalt oxide silica xerogels were microporous and had similar average pore sizes when the seeding ratio is less than 0.4, although the calcined silica seed itself was mesoporous. The microporous structure of the seeded cobalt oxide silica matrices was maintained by the penetration of the polymeric sol into the silica seeds at low seed ratios and effectively anchored the seeds within the polymeric sol–gel network. The best seeded membrane delivered a He permeance of 2.6×10−7molm−2s−1Pa−1 and a He/N2 permselectivity of 97 at 500°C, whilst the permselectivity was only 16 for the non-seeded membrane. It is proposed that the seeding technique gave rise to heterogeneous structures which can effectively block and/or repair large pores and defects of the interlayerless support compared to membranes prepared using a conventional polymeric cobalt silica sol. Ultimately this prevents infiltration of the sol during dip coating and simultaneously assists in the formation of a near defect-free membrane selective layer.

Environment-benign preparation of Ag toughening TiO2/Ti tight ultrafiltration membrane via aqueous sol–gel route

A new aqueous sol was synthesized to prepare Ag-doped TiO2/Ti composite tight ultrafiltration (UF) membranes. This sol was prepared based on water (>95 wt%), for which the hydrodynamic size of 8 nm could be obtained by tuning the electrostatic and steric repulsive forces, breaking the limit of the polymeric sol–gel method to prepare tight UF membranes. For the preparation of the sol, high water content can significantly reduce the use of organic solvents and is more environment-benign. Moreover, Ag doping can considerably improve the toughness of the supported membranes to efficiently prevent the TiO2/Ti composite membranes from cracking and overcome the thermal expansion mismatch of ceramic membranes and their metallic supports. The prepared Ag-doped TiO2/Ti composite membranes presented a molecular weight cut-off of 3 kDa with a pure water flux of 24 L m−2 h−1 bar−1 and exhibited good dye removal capabilities. The proposed Ag doping method has considerable potential for the fabrication of integrated hybrid ceramic–metallic composite membranes.

Gas separation using sol–gel derived microporous zirconia membranes with high hydrothermal stability

A microporous zirconia membrane with hydrogen permeance about 5×10−8mol·m−2·s−1·Pa−1, H2/CO2 permselectivity of ca. 14, and excellent hydrothermal stability under steam pressure of 100kPa was fabricated via polymeric sol–gel process. The effect of calcination temperature on single gas permeance of sol–gel derived zirconia membranes was investigated. Zirconia membranes calcined at 350°C and 400°C showed similar single gas permeance, with permselectivities of hydrogen towards other gases, such as oxygen, nitrogen, methane, and sulfur hexafluoride, around Knudsen values. A much lower CO2 permeance (3.7×10−9mol·m−2·s−1·Pa−1) was observed due to the interaction between CO2 molecules and pore wall of membrane. Higher calcination temperature, 500°C, led to the formation of mesoporous structure and, hence, the membrane lost its molecular sieving property towards hydrogen and carbon dioxide. The stability of zirconia membrane in the presence of hot steam was also investigated. Exposed to 100kPa steam for 400h, the membrane performance kept unchanged in comparison with freshly prepared one, with hydrogen and carbon dioxide permeances of 4.7×10−8 and ~3×10−9mol·m−2·s−1·Pa−1, respectively. Both H2 and CO2 permeances of the zirconia membrane decreased with exposure time to 100kPa steam. With a total exposure time of 1250h, the membrane presented hydrogen permeance of 2.4×10−8mol·m−2·s−1·Pa−1 and H2/CO2 permselectivity of 28, indicating that the membrane retains its microporous structure.

Development of monolithic adsorbent via polymeric sol–gel process for low-concentration CO2 capture

Abstract The objective of this research was to develop a new high-performance CO 2 adsorbent with a simple method. Amine hybrid resorcinol–formaldehyde/silsesquioxane composite aerogel (AH-RFSA) was developed through polymeric sol–gel process and supercritical drying (SCD) technique. CO 2 adsorption performances of AH-RFSA in 1% CO 2 gas mixture (CO 2 + N 2 ) were investigated. A rise in adsorption temperature negatively affects the total CO 2 adsorption capacity of AH-RFSA. However, the useful adsorption capacity does not change with adsorption temperature. Water is more positive to useful CO 2 adsorption capacity than to total CO 2 adsorption capacity. The total CO 2 adsorption capacity achieved with the new adsorbent at 30 °C under dry and humid 1% CO 2 is 3.57 and 4.43 mmol/g, respectively. The useful CO 2 adsorption capacity for complete removal of CO 2 from 1% CO 2 is as high as 3.56 mmol/g. The adsorbent preparation method is inspiring and its resulting adsorbent is exceptional in capacity and regenerability.

Photoluminescence properties of novel Sm3+ and Dy3+ co-activated CsGd(WO4)2 phosphors

CsGd(WO4)2 (CGW) phosphors with different concentration of singly-doped Sm3+ or Dy3+ ions and co-doped Sm3+/Dy3+ ions were prepared through polymeric sol–gel method. The phosphors were characterized by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman and luminescence analysis. The excitation spectra of the phosphors reveal a broad charge transfer band in the short wavelength region and several sharp lines in the longer wavelength region which indicates that these phosphors can be effectively excited by UV and blue LED’s. The CGW: Dy3+, Sm3+ phosphors exhibit the emission peaks in the blue (4F9/2→6H15/2), yellow (4F9/2→6H13/2) and orange–red (4G5/2→6HJ=5/2, 7/2, 9/2) regions under the excitation wavelength of 365nm. The emission spectrum of the co-doped phosphors explains the spread of sensitizing effects from the Dy3+ ions to the Sm3+ ions. All the decay curves are well fitted by the bi-exponential functions. White color chromaticity coordinates (x=0.33, y=0.33) were achieved by optimizing the doping concentration of Sm3+ in Dy3+ doped CGW phosphors.

Process efficacy and novelty of titania membrane prepared by polymeric sol–gel method in removal of chromium(VI) by surfactant enhanced microfiltration

Abstract Micellar enhanced microfiltration (MEMF), a surfactant-based separation process, is promising in removing multivalent metal ions from aqueous solutions. The choice of the membrane is very important in this process. MEMF of chromium(VI) from aqueous solution was studied using cetylpyridinium chloride (CPC) as the cationic surfactant. A simple preparation method for the porous ceramic membrane was developed from nanocrystalline titania (TiO2) powder using polymeric sol–gel process. Further the titania membrane from the powder was sintered at 450 °C for 1 h to obtain a stable structure. The membrane morphology and the crystal structure were analyzed by SEM and XRD. The effect of calcination temperature on membrane porosity was also investigated using mercury porosimetry. The mean pore size, porosity and pure water permeability of membrane were 0.58 μm, 0.32 and 1049 l/(m2 h bar), respectively. These membranes were used in a batch microfiltration system for removal of hexavalent chromium from aqueous solution. Effects of various parameters such as pH, pressure and surfactant to chromate ratio (CPC/Cr) on microfiltration performance were studied. The removal of chromium(VI) was found to be highly dependent on pH of the feed solution. The surfactant to chromate ratio (CPC/Cr) of 2.5 which was below the CMC was found to be ideal for 99% removal of chromium(VI) at initial chromate concentration 100 mg/l. Titania membranes can be effectively used for treatment of chromium containing wastewaters as an alternative. The membrane synthesis and its use in chromate removal are new findings. Other ions can be effectively removed by titania membranes through proper synthetic processes.

Fabrication of mesoporous TiO2 membranes by a nanoparticle-modified polymeric sol process

A straightforward synthesis of crack-free mesoporous titania membrane on a macroporous support without intermediate layers by a nanoparticle-modified polymeric sol–gel process is reported. TiO2 nanoparticle (Degussa P25) was dispersed into the prepared sol to toughen the formed gel. This helped to avoid the cracking of membrane during the drying and early stage of sintering, particularly when the sol–gel transformation occurred on a substrate with an uneven surface. The results of X-ray diffraction and Brunauer–Emmett–Teller analyses show that the nanoparticle doping did not significantly affect the particle size of TiO2 nanocrystals; however, it slightly broadened the pore size distribution because it has a larger particle size compared to the prepared materials. The sols with and without P25 nanoparticle were subsequently dip-coated on a support with average pore size of 150nm, thus formed a mesoporous membrane layer after drying and sintering processes. An integrated, crack-free mesoporous TiO2 membrane layer was obtained by this method, while the membrane prepared with the original sol developed a few cracks. Further, the filtration experiment showed that the prepared membrane had a high flux, and the membrane with nanoparticle modification delivered better separation performance by rejection of dextran.

Effect of the RE (RE = Eu, Er) doping on the structural and textural properties of mesoporous TiO2 thin films obtained by evaporation induced self-assembly method

Polymeric sol–gel route has been used for the preparation of TiO2 and RE2O3–TiO2 (RE=Eu, Er) mesoporous thin films by evaporation induced self-assembly method using Si (100) as a substrate. The influence of the relative humidity (RH) on the preparation of the film has been studied being necessary to work under 40% RH in order to obtain homogeneous and transparent thin films. The films were annealed at different temperatures until 900°C/1h and the anatase crystallization and its crystal size evolution were followed by low angle X-ray diffraction. Neither the anatase–rutile transition nor the formation of other compounds was observed in the studied temperature range. Ellipsoporosimetry studies demonstrated that the thickness of the thin films did not change after calcination at 500°C, the porosity was constant until 700°C, the pore size increased and the specific surface area decreased with temperature. Moreover, the effect of the doping with Er3+ and Eu3+ was studied and a clear inhibition of the crystal growth and the sintering process was detected (by transmission electron and atomic force microscopy) when the doped films are compared with the undoped ones. Finally, Eu3+ and Er3+ f–f transitions were detected by PL measurements.